The present invention relates to torsional vibration monitors, particularly for large rotating machines, such as turbine-generators.
It is known that the rotor of a large machine, such as a turbine-generator, can experience torsional vibrations caused, for example, by phase unbalances in the electrical system supplied by the machine, line switching, or turbine pressure transients. Such torsional vibrations can produce shaft damage, particularly at the location of couplings, journals and seals where lower vibration mode stresses are high.
Moreover, turbine blades can interact strongly with higher frequency modes of rotor torsional vibration, which can cause costly blade damage, particularly in the case of long, low pressure turbine blades. The loss of one or more turbine blades can result in unbalances that produce catastrophic failure of the turbine-generator rotor.
Therefore, torsional vibration measurement is an important procedure during design of such machines. Moreover, since extreme or unforeseen combinations of operating conditions, or changes in shaft configuration, can give rise to dangerous torsional vibration conditions subsequent to construction and installation of such a machine, there is a growing demand for continuous, on-line monitoring of the torsional vibrations of machine.
On-line monitoring can yield data that is useful for detecting a vibration problem and allowing for the possibility of resolving the problem before serious damage occurs. The information provided by such monitoring can be used to effect system modifications, or to provide an estimate of the remaining useful life of the machine, or to provide sufficient warning to an operator if a dangerous situation is eminent.
Analog systems for measuring torsional vibrations at the shaft of such machines have been in use for many years. Typically, such measurements are performed by sensing the passage of the teeth of a turning gear, a toothed wheel at the governor pedestal, or a special toothed wheel at the exciter end of a turbine-generator. In recent years, it has also become possible to take measurements at the tips of a row of turbine blades.
While analog systems have been found to produce useful results, they have certain inherent shortcomings. For example, these systems are not well suited to various noise reduction and signal separation techniques which have been developed and which are easily implemented with digital systems. Analog devices are, moreover, subject to drift and calibration errors.
More recently, digital processing systems have been used for engineering tests. These systems also use sensors, such as magnetic or optical pickups, which can detect the passage of gear teeth, blade tips, or other shaft markings past a selected point. Digital systems do not require that the markings or projections which they sense be equally spaced and can easily incorporate digital processing techniques designed to effect noise reduction and controlled signal extraction. If the data is initially digitized, analog demodulation processes can be eliminated since the data are already in a form suitable for further analysis. Digital procedures are inherently stable and require a minimum of calibration and adjustments.
However, existing digital procedures do have a number of drawbacks, particularly in that they require the processing of a large quantity of data and this, in turn, requires substantial data storage capacity and digitization hardware. Moreover, the large quantity of data which must be processed increases the times required for analysis and communication of data values.
While it may be possible to overcome these drawbacks by providing more and faster digitizers, memory and parallel processors, such solutions would substantially increase the cost of the monitoring system, and this, in turn, would limit its potential use.